Off-resonance effects (e.g., field inhomogeneity, susceptibility, chemical shift) cause artifacts in magnetic resonance imaging (MRI). The artifacts appear as positional shifts along the readout direction in rectilinearly sampled acquisitions. Usually, they are insignificant because of short readout times in normal spin-echo (SE) and gradient-echo (GRE) sequences. However, off-resonance artifacts sometimes appear as severe geometric distortion because of the relatively long readout time in echo planar imaging (EPI).
Over the past decade, spiral imaging techniques have gained in popularity due to their short scan time and insensitivity to flow artifacts. However, off-resonance effects cause blurring artifacts in the reconstructed image. Most spiral off-resonance correction methods proposed to date are difficult to apply to correct for blurring artifacts due to the fat signals, since the fat-water frequency shift is typically much greater than that due to main magnetic field (B0) inhomogeneity across the field of view (FOV). As such, off resonance artifacts remain one of the main disadvantages of spiral imaging.
Currently, off-resonance artifacts due to fat signals are most commonly avoided by use of spatially and spectrally selective radio-frequency (RF) excitation pulses (SPSP pulses) since they excite only water spins, thereby eliminating the off-resonance fat signals and thus avoiding artifact generation. Yet, SPSP pulses may not lead to satisfactory fat signal suppression in the presence of large B0 inhomogeneity. Excitation of only water spins could be achieved through application of chemical shift presaturation pulses [e.g., CHESS pulses] prior to normal spatially selective excitation. However, the effectiveness of these frequency selective RF excitation pulses is dependent on main magnetic field homogeneity.
Alternatively, Dixon techniques have been investigated for water-fat decomposition in rectilinear sampling schemes. In the original Dixon technique, water and fat images were generated by either addition or subtraction of the “in-phase” and “out-of-phase” data sets. Water and fat separation is unequivocal using this technique when magnetic field inhomogeneity is negligible over the scanned object. However, when B0 inhomogeneity cannot be neglected, the original Dixon technique fails to accurately decompose water and fat signals. Therefore, modified Dixon techniques using three data sets (i.e., three-point Dixon (3PD) technique) or four data sets were developed to correct for B0 inhomogeneity off-resonance effects and microscopic susceptibility dephasing. New versions of the Dixon technique use two data sets with B0 inhomogeneity off-resonance correction, i.e., the two-point Dixon (2PD) technique. The water-fat decomposition performance is almost equivalent to that of the 3PD technique, although off-resonance frequency estimation of this technique is unstable for voxels with nearly equal water and fat signal intensities. The advantage of these multiple-point Dixon techniques over spectrally excited RF pulses is that unequivocal water-fat separation can be achieved even in the presence of B0 inhomogeneity. This advantage is of notable importance because neither tissue-induced local magnetic field inhomogeneity nor externally applied magnetic field inhomogeneity can be completely removed.
What is needed is an extension to the 3PD and 2PD techniques to spiral trajectories with effective water-fat decomposition with B0 inhomogeneity off-resonance correction.